What type of reef forms a cicurlar shape located above a guyot?

The type of reef that forms a circular shape above a guyot is called an atoll. Atolls are typically formed from the growth of coral reefs around a submerged volcanic island, which eventually erodes away, leaving a lagoon in the center. Over time, the coral continues to thrive and grow, creating the distinctive ring-like structure characteristic of atolls.